This invention relates to apparatus and methods for continuous injection molding of three-dimensional sheets of thermoplastic material. More specifically a slotted extrusion die is utilized to provide thermoplastic material at a high pressure for injection into cavities in the periphery of a rotating drum mold and simultaneously to provide a substantially uniform backing from which projections extend from one side to provide a three-dimensional layer.
Continuous injection molding apparatus and processes are known in the art. Doleman, et al. disclose such apparatus and processes in U.S. Pat. No. 3,590,109. In such disclosure an extrusion die supplies polymer to rotating molds into pressure zones, i.e. a high pressure zone to allow injection of polymer to the extreme depth of mold cavities in the rotating mold and a low pressure zone to provide polymer in a controlled manner to a surface layer on the rotating mold. Such apparatus requires precisely machined interacting surfaces between the rotating mold and the extruded nozzles, including combinations of lands and grooves to provide three-dimensional sheets having backing layers of parallel rows of ribs between, and interconnecting, parallel rows of projections from one side thereof. Because such backing layers are of such non-uniform geometry, there is generally considerable residual stress in such three-dimensional sheets which makes them not altogether desirable for some otherwise advantageous uses, e.g. in laminates which undergo flexing, such as spray suppression devices.
Moreover such apparatus and methods, as disclosed by Doleman, et al., have the further disadvantage of the need for precise machining of interacting surfaces between the rotating mold and the extrusion die and the requirement for extruding thermoplastic material into separate pressure zones.
Related apparatus and methods are disclosed by Menzin, et al. in U.S. Pat. No. 3,752,619. Such apparatus and methods are similarly disadvantageous in that high and low pressure application of polymer is required to produce three-dimensional sheets.
By this invention applicant has provided apparatus and methods for continuous injection molding of thermoplastic material into a three-dimensional layer which overcomes the aforementioned deficiencies of the prior art.
Accordingly, a principle object of this invention is to provide apparatus and method for continuous injection molding of three-dimensional layers utilizing an extrusion die which can provide polymer at high pressure for injection into cavities and simultaneously providing a backing layer of uniform geometry without uncontrolled flow of polymer in this region of the backing layer.
Another object of this invention is to provide apparatus and methods for continuous injection molding of thermoplastic material into a grass-like three-dimensional layer having blade-like projections extending from one surface of a substantially flat sheet where the three-dimensional layer has substantially low residual stress.
These and other objects of the invention are accomplished by providing apparatus and methods for continuous injection molding comprising a rotatable mold drum having a plurality of cavities extending below a substantially smooth peripheral surface and an extrusion die cooperating with the drum to form an extrusion nozzle therebetween. The cavities have discrete dimensions in the direction of rotation of the drum. The extrusion die has at least one centrally located port for admitting thermoplastic material to the surface of the drum.
The apparatus is arranged so that the substantially flat sheet comprising the backing of the three-dimensional layer is formed and sized in the extrusion nozzle formed between an extrusion face of the die and the rotating mold drum. The extrusion face extends generally parallel to the surface of the drum from at least one port generally centrally located in the die to admit polymer under pressure to the surface of the drum. The extrusion face extends from the at least one port in the direction of rotation of the drum for a distance greater than the discrete dimensions of the cavities; this distance is sufficient that the thickness of a polymer sheet exiting from the extrusion nozzle is substantially the same thickness as the minimum thickness of the nozzle.
Such apparatus is operated so that the extrusion nozzle is smaller than the gap between the other margins around the port and the drum. Surprisingly, such extrusion nozzles provides a seal between the die and the rotating drum to maintain thermoplastic material at high pressure for injection into cavities without allowing the extrusion of thermoplastic material out of the larger gaps between the other larger gaps and the rotating drum.
In preferred embodiments the extrusion die has an arcuate surface matching the radius of the surface of the drum. In other preferred embodiments the at least one port is a channel parallel to the axis of rotation of the drum. The margins around such slot have surface dimensions no less than such discrete dimensions of the cavities to assist in providing a seal for maintaining thermoplastic material under high pressure for injection into the extremities of the cavities.
In preferred embodiments the rotating drum has cooling means to remove thermal energy from thermoplastic material extruded onto the surface and into cavities within the mold drum.
In other preferred embodiments the cavities present a circular opening in the surface of the mold drum and allow for molding of cup shapes on one surface of a substantially flat sheet with a plurality of blade-like elements extending from the ends of the cup shapes.
The apparatus and methods of this invention are useful in the continuous injection molding of three-dimensional layers with projections extending from one surface thereof. Such apparatus and methods are especially useful in the continuous injection molding of a grass-like three-dimensional layer of thermoplastic material having a plurality of elongated blades extending from a substantially flat sheet. Moreover, by the apparatus and methods of this invention such grass-like three-dimensional layers can be produced which have low residual stress, an advantageous property for many applications where such three-dimensional layers undergo flexing, e.g. in spray-suppression devices where grass-like three-dimensional layers are laminated to a substantially rigid backing layer. Moreover, the apparatus and process of this invention can be advantageously combined with laminating methods to apply a sheet of thermoplastic material to the backing surface of three-dimensional layers being formed on the mold drum.